wma: Fix sparse error.

wma_common.c defines wma_log2() with __attribute__ ((const)), but the
declaration in wma.h lacks this attribute. Sparse does not like that
and fails with

	wma_common.c:147:15: error: symbol 'wma_log2' redeclared with different type (originally declared at wma.h:28) - different modifiers

Add the const attribute also to wma.h.